A conventional turbocharger typically relies on a center housing rotating assembly (CHRA) that includes a turbine wheel and a compressor wheel attached to a shaft rotatably supported in a center housing by one or more bearings. During operation, and directly after operation, heat energy from various sources can cause components to reach temperatures in excess of 1000 degrees Fahrenheit (538 degrees Celsius). Sources of heat energy include viscous shearing of lubricant films (e.g., lubricant between a rotating shaft and a bearing surface), viscous heating of inlet gas, exhaust heat, frictional heating, etc. Factors such as mass of the rotating components, lubricant properties, rotational speeds, etc., can affect heat generation.
High temperatures can cause carbonization of carbonaceous lubricants (e.g., oil), also known as coke formation or “coking”. Coking can exasperate heat generation and heat retention by any of a variety of mechanisms and, over time, coke deposits can shorten the lifetime of a lubricated bearing system. For example, coke deposits can reduce bearing system clearances to a point where seizure occurs (e.g., between a bearing and a shaft), which results in total failure of the bearing system and turbocharger. Such phenomenon should be considered during development of turbochargers that operate at high rotational speeds or in high temperature environments and turbochargers with high mass rotating components. For example, use of high strength materials like titanium (e.g., titanium compressor wheels) for rotating components can increase mass of a rotating assembly and thus heat generation.
Various exemplary techniques described herein can reduce coking and, in general, reduce local maximum operational temperatures of a turbocharger's rotating assembly.